The invention relates generally to methods and apparatus for driving loads at high power levels from different power sources, and in particular, to a method and apparatus for driving an induction motor from sources having different voltages and frequencies.
European railway trains, which operate from overhead lines, and in several countries, must operate at several different voltages and frequencies. In current practice, this means that every voltage and frequency requires a separate transformer. These transformers take up nearly the entire engine compartment, and sometimes that of a second vehicle as well, because of the power requirements of the transformer are typically in the three to eight megawatt range.
In particular, traction engines in Germany use an overhead transmission line operating at 16⅔ Hz, at 25 kilovolts. The engine is connected to the source of power by a pantograph which connects to an 8,000 volt-ampere power transformer, rectifier, filter, and three phase inverter which drives the induction traction motors of the train. Interestingly, however, in Germany many of the trains must meet a requirement that the traction engine operate on up to a 4% grade. This relatively steep grade adds weight to German traction engines (due to the larger motor and transformer requirement) and can put them at a severe disadvantage compared to French engines. In particular, the additional weight is sufficient to put German traction engines, which have a weight of 1.9 tons per axle, beyond the 1.7 tons per axle limit for trains allowed to operate in France. Thus, the German engines and hence the German railroad system is at a competitive disadvantage in France since they must use French traction engines to complete some journeys.
It would thus be desirable to provide a solution which reduces the weight of the German engine while at the same time continuing to enable it to operate at voltages and frequencies of different sources. Such a change in the engine compartment of the traction engine would also have equally useful effectiveness in other fields where weight and size are important, and where multiple voltage sources (for example ranging from 480 to 38,000 volts) and frequencies must be accommodated, such as large power supplies and for use with power distribution applications (particularly where the input is constant and different output voltages and configurations are needed). This problem is particularly applicable to the European railway system because of the large cumulative or aggregate size and weight of the transformers needed to operate in various countries, and in particular, as noted above, at substantial grades.
The invention relates, in one aspect, to a method and apparatus for powering a high power load such as an induction motor system having an input of at least 100 horsepower from any one of a plurality of different voltage sources, the AC sources having a frequency in a range of allowable frequencies. The method of the invention features the steps of rectifying an AC input power for generating a first DC output power signal, storing energy at the first DC output power signal using an interlink energy storage circuit having inductive and/or capacitive component(s), modulating the isolated DC output power signal at a high frequency, transforming the high frequency power signal to a selected voltage level, filtering (with or without demodulating) the transformed high frequency signal, and applying the filtered power signal to a load, for example, to operate the at least 100 horsepower induction motor. In some processes where the input to the filter has unwanted modulation, demodulation can be inserted prior to the filtering step; however as used in this application xe2x80x9cfilteringxe2x80x9d is typically meant to encompass the non-linear demodulation process, if present, as well as the typical filter or smoothing process which can involve separate circuitry or be inherent in the load. Also, the transforming step may not be necessary, but if present, can be accomplished by a high frequency, preferably superconducting, transformer. The method is also applicable to non-rail environments wherein the load will typically not be an induction motor and thus may not have the inductive characteristics of the induction motor.
This method is particularly useful because it allows the transforming step, previously performed at a low frequency, to be performed at a much higher frequency. The higher frequency enables the transformer to be of a significantly reduced size. Further, by removing the frequency dependence of the source, by translating the source to a DC signal when the source is an AC signal, only a single transformer may be required for various input frequencies and voltage levels (or a signal source having multiple AC frequencies simultaneously).
The apparatus of the invention features a rectifier circuit for rectifying any AC input power to a first DC output power signal, a storage inductor and/or capacitor for receiving and acting upon the first DC signal, an inverter for receiving the inductor and/or capacitor output and generating a high frequency output signal corresponding to the inductor and/or capacitor output, at a high power; a high frequency power transformer transforming the high frequency power signal to a selected voltage level, a filter (with or without a demodulation capability as noted above) for filtering the transformed signal, whereby the filtered signal can be applied to drive a load, such as the at least 100 horsepower induction motor. As noted above, in non-rail applications, the load need not be an induction motor. In the rail application, however, if ride quality is not a major consideration, motor construction techniques can enable the motor, itself, to act as the filter (to remove high frequencies). Also, in future, other motor topologies, such as synchronous motors, may be used in rail applications.
Preferably, the invention is capable of supplying greater than 100 watts, and in particular, greater than 50,000 watts of power. Preferably, also, portions of the components are actively cooled to at least a temperature below the dew point of the surrounding air, and in a protected environment. The advantages of the invention are increased when the cooled components are cooled to less than 230 K, preferably less than 180 K and most preferably, 150 K.